Adversaries may use an existing, legitimate external Web service as a means for sending commands to a compromised system without receiving return output over the Web service channel. Compromised systems may leverage popular websites and social media to host command and control (C2) instructions. Those infected systems may opt to send the output from those commands back over a different C2 channel, including to another distinct Web service. Alternatively, compromised systems may return no output at all in cases where adversaries want to send instructions to systems and do not want a response. Popular websites and social media acting as a mechanism for C2 may give a significant amount of cover due to the likelihood that hosts within a network are already communicating with them prior to a compromise. Using common services, such as those offered by Google or Twitter, makes it easier for adversaries to hide in expected noise. Web service providers commonly use SSL/TLS encryption, giving adversaries an added level of protection.
ESXi shell or scheduled tasks initiating outbound HTTPS to known public services without inbound return or loggable response, used to fetch instructions.
Suspicious process initiating outbound connections to web services without corresponding response or return traffic, indicative of one-way command channels.
Process using URLSession or similar API to fetch from web services without any response handling, indicative of one-way C2 channels.
Restrict Web-Based Content
Restricting web-based content involves enforcing policies and technologies that limit access to potentially malicious websites, unsafe downloads, and unauthorized browser behaviors. This can include URL filtering, download restrictions, script blocking, and extension control to protect against exploitation, phishing, and malware delivery. This mitigation can be implemented through the following measures: Deploy Web Proxy Filtering: - Use solutions to filter web traffic based on categories, reputation, and content types. - Enforce policies that block unsafe websites or file types at the gateway level. Enable DNS-Based Filtering: - Implement tools to restrict access to domains associated with malware or phishing campaigns. - Use public DNS filtering services to enhance protection. Enforce Content Security Policies (CSP): - Configure CSP headers on internal and external web applications to restrict script execution, iframe embedding, and cross-origin requests. Control Browser Features: - Disable unapproved browser features like automatic downloads, developer tools, or unsafe scripting. - Enforce policies through tools like Group Policy Management to control browser settings. Monitor and Alert on Web-Based Threats: - Use SIEM tools to collect and analyze web proxy logs for signs of anomalous or malicious activity. - Configure alerts for access attempts to blocked domains or repeated file download failures.
Network Intrusion Prevention
Use intrusion detection signatures to block traffic at network boundaries.